The present invention relates to an internal combustion engine and more particularly to a variable valve timing system for an internal combustion engine.
Exhaust gas recirculation (EGR) is widely employed to reduce nitrogen oxides (NOx) emissions from internal combustion engines. EGR can be accomplished either externally or internally. The external method involves diverting the exhaust gases from some point in the exhaust system back to the intake system. To reduce NOx emissions to a certain sufficiently low level, the amount of EGR must be increased considerably. Increasing the flow rate of the exhaust gases through the intake system to meet the demand above will increase the deposition of carbon on the interior wall of the intake system and EGR system to such an degree as to cause deterioration of the induction efficiency of the engine and the reduction efficiency of NOx removal. To alleviate this problem, it has been proposed to increase the internal EGR to suppress the external EGR. All naturally aspirated engines have some inherent EGR; that is, some portion of the products of combustion remain in the cylinder (residual gas) which mix with the incoming fresh charge before combustion is initiated. The amount of residual gas can be controlled by increasing or decreasing the valve overlap period during which both inlet and exhaust valves are open.
As well as having the potential for reducing Nox, the variation of valve overlap has a direct effect on engine combustion and power output. Large valve overlap enhances power output at high-speed wide-open throttle operation, while low-speed part-throttle operation requires low overlap for smooth running. These interrelationships indicate that a variable valve timing system capable of adjusting valve overlap while running may be desirable for optimizing engine operation for both emissions and performance.
To accomplish this, a known variable valve timing system comprises a cam shaft axially movable by an actuator. The cam shaft has a cam to operate a valve through a rocker arm or lever. The cam has a plurality of valve control tracks each associated with a different range of engine operation. The changeover of the system from one track to another is effected by axial translation of the cam shaft by the actuator. Because the cam shaft is driven by the engine to rotate at high speeds, there exist some difficulties to axially move the cam shaft by the actuator. Because the cam must have different valve control tracks, it is not easy to manufacture. Therefore this known system is complicated in construction.